1. Field of the Invention
The present invention relates to an optical device, and more particularly to an optical device for reading an image, which is used in a facsimile apparatus, image scanner, LED printer or the like. Specifically, the present invention relates to an optical device for forming an image of an object on a light-receiving surface at a magnification of 1:1.
2. Description of the Related Art
For a facsimile apparatus, image scanner, LED printer or the like, an optical device is used in which an image on a line is formed on a sensor or a photoreceptor drum at a magnification of 1:1.
Known examples of these optical devices include a device in which an array of rod lenses made of cylindrical transparent bodies having a refractive index distribution and an array of convex lenses are stacked in three layers (See, for example, Japanese Examined Patent Publication No. SHO 49(1974)-8893, Japanese Unexamined Patent Publication No. SHO 57(1982)-104923, and Japanese Unexamined Patent Publication No. SHO 57(1982)-66414) and a device in which arrays of thick convex lenses are stacked in two layers (See, for example, Japanese Unexamined Patent Publication No. SHO 55(1980)-83005).
Also, a device in which two cylindrical lens arrays and two cylindrical lenses are used (See, for example, Japanese Unexamined Patent Publication No. HEI 6(1994)-208081) and a device in which a cylindrical mirror, a lens and a cylindrical lens are used (See, for example, Japanese Unexamined Patent Publication No. SHO 58(1983)-34419) are known.
In an optical device used in a facsimile apparatus or an image scanner for reading an image, high utilization efficiency of light is desired for saving power and, at the same time, a large image size is desired for increasing the mounting tolerances.
However, in such a conventional optical device, when an attempt is made to decrease the F number by enlarging the aperture of the lens or by decreasing the working distance in order to increase the image size and to gain the image brightness, integration of the refraction angle of the light beam passing through a peripheral portion of the lens increases, so that the aberration increases unfavorably.